1. Field of the Invention
The invention relates to an apparatus for sensing the presence or absence of liquid at various predetermined levels within a vessel.
2. State of the Art
U.S. Pat. No. 3,520,186 teaches a single point sensing system employing a continuous signal and providing a simple on-off response. The system comprises means for continuously transmitting an ultrasonic signal from one piezoelectric transducer to another and means for amplifying any signals that have more than a certain critical amplitude at the time of reception. When gas occupies space between the transducers, transmitted signals do not reach the receiving transducer with sufficient strength to be amplified and, hence, detected.
A unitary structure for containing multiple sensing points of the above patent is not taught in the prior art. Interference between multiple sensing points as the result of receivers picking up signals from an incorrect transmitter or signals carried through the walls of the structure prevents simply joining a number of sensing points of the above patent in close proximity. Mounting a single sensing point of the above patent on a slideable member allows measurement at multiple level points but such moving parts introduce lubrication and wear problems. Misalignment and jamming of the moving parts are also a problem in applications such as in railroad tank cars in which the sensor is subjected to jousling and vibration.